1. Field of the Invention
Exemplary embodiments of the present invention relate to a pre-distortion apparatus and a method thereof; and, more particularly, to a pre-distortion apparatus for minimizing distortion of an output signal output from a power amplifier and a method thereof.
2. Description of Related Art
In wired/wireless communication system, a transmitter amplifies a power of a transmission signal to be reached at a receiver because the transmission signal is attenuated in a wired/wireless channel. The transmitter commonly employs a power amplifier for amplifying the power of the transmission signal.
The power amplifier must maintain linearity of an input signal and an output signal to improve efficiency. However, the power amplifier has a non-linearity property.
Many methods have been introduced to linearize the non-linearity property of the power amplifier. Among them, a digital pre-distortion method was introduced to improve the non-linearity property of the power amplifier. The digital pre-distortion method distorts an input signal in advance to linearize a relation between an input signal and an output signal of a power amplifier.
Particularly, the digital pre-distortion method includes a complex gain type digital pre-distortion method and a polynomial type digital pre-distortion method. The complex gain type digital pre-distortion method calculates a complex correction value according to a magnitude of an input signal by adaptively comparing the input signal with a non-linear distorted signal and corrects the input signal in real time based on the complex correction value. The polynomial type digital pre-distortion method approximates a non-linear property of a power amplifier to a polynomial expression, adaptively calculates a coefficient of the polynomial expression from input and output signals, calculates an inverse transfer function of the power amplifier, and corrects a transmission signal based on the inverse transfer function.
A complex gain type digital pre-distortion apparatus provides a linearization method when a property of a power amplifier is not changed due to a constant bias voltage. Lately, a high power amplifier is commonly used. The high power amplifier has a bias voltage dynamically changing according to an input signal. Accordingly, the property of the high power amplifier is changed according to a magnitude of an input signal. Therefore, it is difficult to linearize the high power amplifier using the digital pre-distortion apparatus.
When a communication system employs a pre-distortion apparatus with a high power amplifier having a bias varying according to an input signal, a bias voltage of the high power amplifier is changed while updating a complex correction coefficient. Therefore, a convergence property of an adaptive algorithm is deteriorated.